Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central nervous system. GABA receptors are heteromeric, and are divided into three main classes: (1) GABAA receptors, which are members of the ligand-gated ion channel superfamily; (2) GABAB receptors, which may be members of the G-protein linked receptor superfamily; and (3) GABAC receptors, also members of the ligand-gated ion channel superfamily, but their distribution is confined to the retina. Benzodiazepine receptor ligands do not bind to GABAB and GABAC receptors. Since the first cDNAs encoding individual GABAA receptor subunits were cloned the number of known members of the mammalian family has grown to 21 including α, β, and γ subunits (6α, 4β, 4γ, 1δ, 1ε, 1π, 1θ, and 3ρ).
A characteristic property of GABAA receptors is the presence of a number of modulatory sites, one of which is the benzodiazepine (BZ) site. The benzodiazepine binding site is the most explored of the GABAA receptor modulatory sites, and is the site through which benzodiazepine-based anxiolytic drugs exert their effect. Before the cloning of the GABAA receptor gene family, the benzodiazepine binding site was historically subdivided into two subtypes, BENZODIAZEPINE1 and BENZODIAZEPINE2, on the basis of radioligand binding studies on synaptosomal rat membranes. The BENZODIAZEPINE1 subtype has been shown to be pharmacologically equivalent to a GABAA receptor comprising the α1 subunit in combination with a β subunit and γ2. It has been indicated that an α subunit, a β subunit and a γ subunit constitute the minimum requirement for forming a fully functional Benzodiazepine/GABAA receptor.
Receptor subtype assemblies for BZ-sensitive GABAA receptors include amongst others the subunit combinations α1β2/3γ2, α2β2/3γ2, α3β2/3γ2, α4β2/3γ2, and α5β2/3γ2. Subtype assemblies containing an α1 subunit (α1β2γ2) are present in most areas of the brain and are thought to account for 40-50% of GABAA receptors in the rat. Subtype assemblies containing α2 and α3 subunits respectively are thought to account for about 25% and 17% of GABAA receptors in the rat. Subtype assemblies containing an α5 subunit (α5β3γ2) are expressed predominately in the hippocampus and cortex and are thought to represent about 5% of GABAA receptors in the rat. Two other major populations are the α2β2/3γ2 and α3β2/3γ2 subtypes as stated above. Together these constitute approximately a further 35% of the total GABAA receptor population. Pharmacologically this combination appears to be equivalent to the BENZODIAZEPINE2 subtype as defined previously by radioligand binding, although the BENZODIAZEPINE2 subtype may also include certain α5-containing subtype assemblies.
The present pharmacology of agonists acting at the BZ site of GABAA receptors suggests that α1 containing receptors mediate sedation, anticonvulsant activity, ataxia, and anterograde amnesia, while α2 and/or α3 GABAA receptors mediate anxiolytic activity. α5 containing GABAA receptors are involved in memory functions (U. Rudolph et al., Nature 1999, 401, 796; K. Löw et al., Science 2000, 290, 131; McKernan Nature Neurosci. 2000, 3, 587; F. Crestani et al., Proc. Nat. Acad. Sci. USA 2002, 99, 8980; M. S. Chambers et al., J. Med. Chem. 2003, 46, 2227).
It is believed that agents acting selectively as benzodiazepine agonists at GABAA/α2, GABAA/α3, and/or GABAA/α5 receptors possess desirable properties. Compounds which are modulators of the benzodiazepine binding site of the GABAA receptor by acting as benzodiazepine agonists are referred to hereinafter as “GABAA receptor agonists.” The GABAA/α1-selective (α1β2γ2) agonists alpidem and zolpidem are clinically prescribed as hypnotic agents, suggesting that at least some of the sedation associated with known anxiolytic drugs which act at the Benzodiazepine 1 binding site is mediated through GABAA receptors containing the α1 subunit. Recently, two studies have shown that the majority of additive properties of diazepam are mediated by α1 subtypes (N. A. Ator et. al., J. Pharm. Exp. Thera. 2010, 332, 4-16; K. R. Tan et. al., Nature, 463, 769-774).
It is also known that some benzodiazepine derivatives, such as QH-ii-066, bind with high affinity to GABAA/α5 receptors (Ki<10 nM), intermediate affinity to GABAA/α2 and GABAA/α3 (Ki<50 nM), and poorer affinity to GABAA/α1 receptors (Ki>70 nM), unlike diazepam which binds with high affinity to all four diazepam-sensitive GABAA receptors (Ki<25 nM), as disclosed in Huang, et al., J. Med. Chem. 2000, 43, 71-95. However, such benzodiazepine derivatives may contain ester linkages, and may thus be sensitive to hydrolysis in vivo (e.g., by esterases). What is needed are GABAergic receptor subtype selective ligands that lack ester linkages, and are less sensitive to hydrolysis in vivo by esterases.